1. Field of the Invention
The present invention relates mainly to an image sensing apparatus which utilizes a flashlight device.
2. Brief Description of the Related Art
In an image sensing apparatus such as an electronic still camera, an employment of an electronic flashlight device has been indispensable up to now when a subject illuminated by a low intensity light is sensed an image. Because a period for sensing the image in the electronic camera is an 1/60 second if a specification of the image is, for instance, of the National Television System Committee (referred to as “NTSC”) Standard, an accumulation time of an image sensing device (sensor) is made coincident in general with the image sensing period in many cases. However, sensing the images is also capable either in a comparatively longer period or in a comparatively shorter period than that of the NTSC Standard. The image sensing period described herein corresponds to a so called shutter speed of a silver halide film camera. When the electronic flashlight device is employed to the electronic still camera, discharging the flashlight lamp is required to be carried out within the period for sensing the image. To satisfy the condition mentioned above, the flashlight emission has to be performed synchronously with a synchronous signal superimposed on an image signal.
When a high shutter speed is used in the silver bromide film camera employing a focal plane shutter, a post curtain of the shutter begins closing before a fore curtain of the shutter completes opening. To provide a film with a uniform exposure light on that occasion, a flashlight device which can emit a flat light emission has been invented. The flat light emission herein means an emission which performs a continuous flashlight emission to enable obtaining a stable light emission for a certain period of time.
FIG. 9 is a block diagram showing a constitution of a prior image sensing system seen in a conventional electronic still camera equipped with above-mentioned flashlight device. In the figure, a numeric sign 1 stands for a lens for sensing the image, 2 stands for the image sensor, 3 stands for a signal processing circuit (processor) which generates the image signal from an output signal produced from the image sensor 2, 4 stands for a timing generator (referred to as “TG”) which controls accumulating and reading-out behaviors of electric charges in the image sensor 2, 5 stands for the electronic flashlight device and 6 stands for a stroboscope (a prior trade name of an electronic flashlight tube) control device which instructs the electronic flashlight device 5 to initiate the discharging by applying a flashlight emission initiating signal S1 to the flashlight device 5.
In the electronic still camera constituted as mentioned above, rays of light incident through the lens 1 into the image sensor 2 are transformed photo-electrically into electric charges and accumulated herein. When the timing generator 4 applies a pulse signal for reading-out the electric charges to the image sensor 2, the accumulated charges are transferred to the signal processing circuit 3, from which the transferred charges are transmitted as the image signal. Herein the stroboscope control circuit 6 controls so that the flashlight device 5 emits the flashlight during the charge accumulating period of the image sensor 2.
FIG. 10 is a view for illustrating a timing of aforesaid flashlight emission emitted from device 5 and the timing of aforesaid image signal, which shows the charge accumulating period of the image sensor 2, the light emission quantity of the flashlight device 5, the pulse signal for reading-out the charges from the image sensor 2 and an output image signal.
As shown in the figure, the image sensor 2 initiates the charge accumulation at a timing of T1, the accumulated charge quantity is read-out at a timing of T2 and the light emission quantity emitted from the electronic flashlight device 5 is controlled so that the flashlight emission is completed between the timing of T1 and the timing of T2. The signal charges which are read-out at the timing of T2 are transmitted as the image signal during the period between the timing of T2 and the timing of T3.
The employment of the stroboscopic flashlight, which serves as a means for supplementing the light quantity when a subject illuminated under a low intensity light or under a rear light is sensed the image, has so generally accepted in a silver salt film camera that the flashlight device is not only capable of being equipped but also built-in into a popular type camera and even into a returnable camera which is called a “single use camera” in US and a “film with a lens” in Japan, respectively. Especially in recent years, applications of the stroboscopic lamps have been widespread even to the electronic still camera and even to a camcoder.
Among the stroboscopic flashlight devices, there is one having a variable illuminating angle called a “zoom stroboscopic device”. The control of the illuminating angle in the zoom stroboscopic devices is performed in general in accordance with a focal distance of the lens. If the focal distance of a zoom lens is, for instance, shortened toward a wide angle side, the illuminating angle is controlled to be enlarged so that the stroboscopic light can illuminate all the area sensed the image. If the focal distance of the zoom lens is lengthened toward a telephoto side on the contrary, the illuminating angle is controlled so as to be small.
FIG. 11 is a view showing a focal position of the lens and an illumination-capable area of the stroboscopic lamp. In the figure, W1 indicates the image sensing area when the focal distance of the lens is F1 while W2 indicates the image sensing area when the focal distance of the lens is F2, respectively. The stroboscopic tube used herein can illuminate up to the image sensing area W1 of which illumination area is represented by S.
However, when the shutter speed is fast and the accumulation time of the image sensor is short in the image sensing apparatus utilizing aforesaid conventional flashlight device, many cases where the flashlight emission timing of the flashlight device does not coincide with the shutter timing or the flashlight emission period exceeds the exposure time have frequently taken place. Consequently, there has been a problem that an adequate exposure cannot been attained.
On the other hand, there exist extremely many variations in species of lens because it is required to cope with various situations for sensing the image. Even in the zoom lens, for instance, there are a wide angle-sided lens of which focal distance varies between 28 and 80 mm and a telephoto-sided lens of which focal distance varies between 100 and 300 mm. Those situations are similar to the zoom stroboscopic devices.
As a result, in the camera which can exchange either the lens or the stroboscopic lamp, there arises a problem that the illuminating range of the flashlight device cannot cover the image sensing area, depending on a combination manner of the lens and the stroboscopic lamp to be used.
Namely, if the focal distance is F1 as shown in FIG. 11, the stroboscopic lamp illuminates all of the subject located within the image sensing region W1 as expected. If the focal distance turns to F2, the lamp can only illuminate partially the illumination range S located within the image sensing area W2. Consequently, peripheral portions of the sensed image turn dark. Accordingly, a camera user has always to be aware of both the focal distance of the lens and the illuminating angle of the stroboscopic lamp which are to be mounted, respectively, during sensing the image.